This invention relates to printing plates, and, more particularly, to a flexible photosensitive printing medium deposited upon a substrate to produce a printing plate.
Flexographic printing is widely used in the production of newspapers and in the decorative printing of packaging media. In flexographic printing, a layer of a flexible printing medium is coated onto a flexible substrate such as a thin sheet of steel, aluminum or synthetic polymer, to form a printing plate. A relief pattern corresponding to the negative of the image to be printed is formed in the printing medium. The plate is then mounted on the printing press, and the printing commences.
One type of printing medium is natural or synthetic rubber. This printing medium has excellent mechanical properties, but the preparation of a printing plate with a rubber printing medium is labor intensive and slow. A pattern plate and a matrix board are prepared, and rubber plates are then hot press molded. Molded rubber printing media are not practical for printing applications with short deadlines, such as newspapers. Due to the nature of the medium and the imaging techniques described above, photosensitive printing plates produce printed images of a generally superior nature to those of molded rubber media.
In another approach, the printing medium is formed of an elastomeric photopolymer composition. A layer of the photosensitive material is coated onto the substrate to form the printing plate. The coated side is exposed with light through a photographic negative of the image to be printed, causing photopolymerization of the exposed portion of the printing medium, which then becomes physically hardened and resistant to removal in a solvent. The unexposed and therefore unhardened portion of the printing medium is removed by washing in a solvent, leaving a relief pattern of the image to be printed. The printing plate is mounted on a press and printing is commenced.
Photosensitive printing plates fall into two broad categories, liquid compositions and solid compositions. The liquid compositions require the actual manufacture of the relief printing surface from a viscous liquid calendared at the printing site on complex machinery designed for that purpose. Solid photopolymer printing plates have the significant advantage of being pre-manufactured and therefore simpler and more reliable to use. Flexographic printing using photosensitive solid printing media offers the desirable combination of fast, inexpensive processing and long press runs.
There are several types of photosensitive, solid flexographic printing plates. Plates using modified thermoplastic elastomeric rubber or rubber-like printing media have excellent mechanical properties, but can be processed only In organic solvents. See, for example, U.S. Pat. Nos. 4,359,246 and 4,622,088. Plates using acrylic modified polyolefinic copolymer latex printing media are less elastic and flexible, but can be processed using water as the solvent. See, for example, U.S. Pat. Nos. 4,275,142 and 4,927,738. Plates using elastomeric copolymers with carboxyl groups in the printing medium have good mechanical properties but must be processed in aqueous alkaline mixtures of water and water-soluble organic compounds. These solvents suffer from many of tile same disadvantages as fully organic solvents.
The need to use organic solvents for processing photosensitive, solid printing plates other than the acrylic modified polyolefinic copolymer printing medium is a major obstacle in their utilization. Such solvents include, for example, methyl ethyl ketone, benzene, xylene, carbon tetrachloride, 1,1,1-trichloroethane, and trichloroethylene, alone or in combination with a cosolvent such as ethanol or isopropyl alcohol. The organic solvents present problems of adverse health effects to exposed workers, disposal without environmental damage, and risk of fires. The one type of printing medium that does not require washing in an organic-containing solvent, the acrylic modified polyolefinic copolymers, has insufficient elasticity and toughness for some printing applications such as flexible packaging.
There exists a need for a photosensitive solid printing medium and plate that combines excellent physical properties with the ability to be washed in water without the use of alkaline additives or any organic solvents. The present invention fulfills this need, and further provides related advantages.